Generally, the most commonly encountered fluorescent lamps are the so-called "preheat" and "rapid-start" types of fluorescent lamp. In the "preheat" type of lamp, heater current flows through the lamp electrode during lamp ignition. Thereafter, an external voltage sensitive starter opens the electrical circuit to the lamp electrode and heater current flow is discontinued. The "rapid-start" type of fluorescent lamp normally has a constant flow of heater current through each electrode not only during ignition but also during operation of the lamp. However, heater current flow during operation, as in the "rapid-start" lamp, is lost power which undesirably reduces the operational efficiency of the lamp.
Numerous suggestions have been made for enhancing the efficiency of "rapid-start" type fluorescent lamps. For example, U.S. Pat. Nos. 4,052,687; 4,097,779; 4,114,968; 4,156,831 and 4,171,519 which are all assigned to the Assignee of the present application, provide numerous configurations for enhanced operation of "rapid-start" type fluorescent lamps. Primarily, each provides a thermally responsive circuit breaker suitable for discontinuing heater current during operation of the fluorescent lamp.
However, it has been found that problems still exist in the fabrication of enhanced rapid-start fluorescent lamps in spite of the above-enumerated advantages of such improved configurations. For example, it is known that the electrode or filament for a rapid-start fluorescent lamp is provided with an emissive coating of alkaline earth oxides and that this electrode requires current passage therethrough in order to activate and outgas the coating. Also, the lamps wherein the electrode is located are relatively hot from the necessary hot sealing process. Moreover, this relatively hot temperature is sufficient to open the bimetal switch of the circuit breaker therein which, in turn, would ordinarily prevent the desired current flow for activating the electrode or filament coating. However, the addition of a meltable by-pass element permits this electrode coating activation even though the bimetal switch is open and upon completion of the electrode material activation is removed as by melting for instance.
Also, the above-mentioned circuit breaker within a rapid-start lamp represents but a small portion of the cost of the lamp but can easily render the whole lamp useless if defective. For example, a broken, missing or improperly attached meltable by-pass element will prevent activation of the electrode coating and result in catastrophic failure of the rapid-start lamp. Thus, it is a highly desirable condition that the circuit breaker can be tested prior to inclusion within the rapid-start lamp.
Previously, the meltable by-pass element in circuit breakers for rapid-start fluorescent lamps was fabricated from a very low cold resistance molybdenum wire. Such a wire was selected as a compromise material which would pass sufficient current to insure activation of the electrode coating and still consistently and reliably melt or burn open in response to an energy pulse provided by a discharging capacitor. However, this very low cold resistance of the meltable by-pass element (about 30 milliohms or less for 0.002-inch diameter molybdenum wire) makes automatic inspection and resistance measurement thereof a formidable problem.